As organisms age there is often decrease in strength and vigor which is reflected in the organism's physical and metabolic activities. Rats, no less than humans, seem to exhibit these characteristics of increasing age. In an effort to examine the biochemistry of some of these changes, male Fisher 344 rats of ages varying from two to twenty-six months were sacrificed and tissues were removed for study. Serum samples from the rats were deproteinized and the free amino acid content was determined. No trend was evident in the concentration of any amino acid even though there was considerable variation from rat to rat. On average, the concentration of any given amino acid was found to be the same in very old rats as it was in very young ones. Several enzyme activities were measured in liver homogenates. Carbonyl reductase (using metyrapone as a substrate) gradually declined with age whereas lactate dehydrogenase remained generally constant. Alkaline phosphatase and thiolase were low in young animals and rose as they reached middle age (until they were about 12-16 months old) and then declined again as the animals became really old. Fatty acid, measured in liver homogenates, were found to increase to middle age and then to decline. Linolenic acid, however, did not vary much with age.